Many radio frequency (“RF”) communication systems will utilize a control channel to assign subscriber units to working channels. These systems may make use of time division multiple access (“TDMA”) and frequency division multiple access (“FDMA”) technologies, sometimes mixing both within a single system.
Today, it is desired to have a single control channel assign a subscriber unit to either a TDMA working channel and/or a FDMA working channel, while minimizing access time to operate on the working channel. FIG. 1 illustrates a system diagram of a typical communication system 100. The communication system 100 comprises at least one site 102 and at least one subscriber unit 104. In the example illustrated in FIG. 1, the site 102 comprises a plurality of base radios 1061, 1062, and 1063. In this example, a first base radio 106, provides a control channel service, a second base radio 1062 provides a TDMA working channel service, and a third base radio 1063 provides a FDMA working channel service. The subscriber unit 104 may communicate with any single base radio 106 depending on its desired service. It should be noted that while FIG. 1 depicts only two sites, one subscriber unit, and three base radios, a practical system may include additional sites, subscriber units and base radios.
Synchronization between the control channel and the working channel is required in order to transmit and receive information on the working channel. In the case of the TDMA working channel, a subscriber unit 104 must learn the TDMA frame/slot boundaries to avoid overlapping transmissions.
One approach that utilizes learning the synchronization solely from the working channel faces challenges: increased access time and/or audio delay for initial transmissions. Consider a subscriber unit 104 that has been granted to transmit in one slot of a given frequency. That subscriber unit 104 must first tune to the transmit frequency of the working channel and wait for the frame synchronization pattern. Similar to FDMA channels, the frame synchronization pattern may be transmitted only once every 180 ms per slot during voice transmissions. The subscriber unit 104 can use the frame synchronization pattern located in slot one, slot two, etc. regardless of the designated slot to gain the ability to decode symbols on the outbound channel.
Acquiring outbound bit synchronization, however, is not adequate to discover the slot number. In order to discover which slot is slot one, slot two, etc., the subscriber unit 104 must decode at least one slot's worth of information. Decoding at least one slot's worth of information may impair the subscriber unit 104 from being ready to transmit in the next useable inbound slot.
Previous systems either use a TDMA control channel to assign a subscriber unit 104 to a TDMA working channel and use a FDMA control channel to assign a subscriber unit 104 to a FDMA working channel, or attempt to synchronize FDMA data units to TDMA frame boundaries to assign a subscriber unit 104 to a TDMA working channel 206, as described by U.S. Pat. No. 6,014,375 of Janky issued Jan. 11, 2000, titled “TDMA Radio Protocol with Adaptive Vocoder Selection” (hereinafter referred to as the '375 patent).
As illustrated in FIG. 2, and as described in the '375 patent, every eight data units 200 in a control channel 202 is synchronized in time to every five TDMA frames 204 in the working channel 206. One disadvantage, however, to the '375 patent is that the data units 200 on the control channel 202 must be of a fixed length in order for the control channel 202 to assign a subscriber unit 104 to the working channel 206; thus, the '375 patent does not account for synchronizing a control channel 202 having variable length data units 200 with a TDMA working channel 206.
Additionally, it is also desired to have air interface encryption synchronization between the control channel 202 and the working channel. The current state of the art systems utilize a message transmitted periodically on a control channel 202 to provide encryption synchronization for the same control channel 202 as described by U.S. Pat. No. 5,502,767 of Sasuta et al. issued Mar. 26, 1996, titled “Method for Maintaining Secure Information Synchronization on a Control Channel” (hereinafter referred to as the '767 patent). A disadvantage to the '767 patent is that separate encryption synchronization is required for both the control channel 202 and the working channel 206, which introduces an additional delay into the system when the subscriber unit 104 switches between the control channel 202 and the working channel 206.
Further, FIG. 3 illustrates a typical configuration where the control channel 202 is asynchronous to a FDMA working channel 300. The asynchronous nature between the channels makes it more difficult to obtain encryption synchronization between the control channel 202 and the working channel 206/300 when the subscriber unit 104 switches between the control channel 202 and the working channel 206/300.
Thus, there exists a need for a single control channel 202 to assign a subscriber unit 104 to both a TDMA and/or a FDMA working channel, while minimizing access time to operate on the working channel and while providing an air interface encrypted service.